In the manufacture of glass, glass forming ingredients are usually melted and refined in a glass melting furnace having a melting zone and a fining zone. The glass forming ingredients generally are heated by a plurality of air-fired burners oriented within the furnace so as to provide heat to the ingredients and to the resulting melt, which flow from the melting zone to the fining zone. A pool of the molten glass or melt is always maintained to operate the melting process in a continuous manner. When a certain amount of the melt is discharged from the furnace through outlet means, a certain amount of the glass forming ingredients is added to replenish the furnace with the melt. Unmelted, solid glass forming ingredients, having a lower specific gravity than the melt, float on the surface of the melt. As the melt is discharged through the furnace throat or other outlet means, the solid glass forming ingredients near the furnace throat area can descend from the surface of the melt to enter the furnace throat, thereby exiting with the melt. The presence of these unmolten glass crystals in the discharged melt adversely affects the quality of a glass product which may be bottles, fibers or other products.
The prior art has addressed this problem in various ways. U.S. Pat. No. 4,816,056 (Tsai et al) issued on Mar. 28, 1989, discloses impinging a transversely extending curtain of flames onto the surface of the molten glass in an area between the discharging end and the entrance end with sufficient force to influence the surface to direct the unmelted, solid glass forming ingredients away from the discharging end. The flames may be supplied by oxy-fuel burners and can be impinged at an angle of 5.degree. to 20.degree..
U.S. Pat. No. 4,473,388 (Lauwers) issued Sep. 25, 1984 discloses melting unmelted glass forming ingredients in the melting zone by directing the tips of oxy-fuel flames at the interface of the solid glass forming ingredients and the molten glass. This flame barrier melts a substantial amount of the glass forming ingredients. However, some solid, glass forming ingredients escape from this barrier.
U.S. Pat. No. 3,592,623 (Shepherd) issued Jul. 13, 1971, discloses circulating unmelted glass forming ingredients in whirlpool fashion at the entrance end of the melting zone, thereby preventing the movement of the ingredients toward the firing zone. This whirlpool like circulation is accomplished by directing the flames of oxygen-fuel burners toward a hot spot. The term "hot spot" refers to a location in the furnace where the temperature of the roof is maximum (about 1580/C). At both ends of the furnace, the roof temperature is lower, (the entrance end about 1350.degree. C. and the outlet end about 1480.degree. C.). This "hot spot" location normally corresponds to an area where the surface covered by solid floating glass forming ingredients (melting area) changes into a surface almost free from the floating glass forming ingredients (refining area).